Reorganise global admin styles

html { ... } in document.scss doesn't make any sense.
Redesign admin edition show page

This is the first pass at redesigning the edition show page to
give it more structure. The main change is you can no longer
preview documents from the admin – you have to go through to
the frontend preview page. This is because the admin preview is
misleading – you aren't actually previewing how it will look.

Some other notes

- Edition#previewable? is now just `not published?`.
  There was an odd concept of previewable before.
- I have started to clean up some of the admin CSS to match how
  it is done on the frontend. This will continue, and there is
  more cleaning to come.

https://www.pivotaltracker.com/story/show/53973095